TUBE AMP CORNER
What is Bias & Why is Bias important?
David Brand alias The G-man
Copyright 2001 - Cosmic Debris Musicians Magazine
One of my pet peeves (not that You care) is a person who is armed with only enough information to be
dangerous and annoying. This is probably because I quite often feel that way about myself.
With that in mind the subject of today's little story is Bias and the tube amplifier.
What is Bias?
1. A negative voltage applied to the grid of a tube in order to control electron flow.
2. The amount of current a tube draws at idle.
3. A signal on a scope which shows a crossover notch reflecting distortion.
4. A quick way for the amp guy to make $40.
All of these answers are partly true. If any of this makes sense to you other than #4 go to part B
of this test
Why is Bias important?
1. If there is too much electron flow the tube will become saturated.
2. If incorrectly adjusted the amp will run too hot or too cold, resulting in poor performance or
premature failure of the tubes or even the amp.
3. Tubes are rated for to operate at optimum conditions by horrendous math calculations that You or I
don't want to know about.
4. The amp guy needs the $40.
Seriously now, back in the days when tubes were being developed the engineers would do all kinds of
tests and math and graphs to determine the best operating conditions for a particular tube.
This info is printed in tube manuals and includes maximum plate voltage, suggested grid voltages and
maximum plate dissipation. Using these figures amp designers arrive at what voltages to use in their
Some amp manufactures have adjustable bias while some amp makers don't. I read an article by
one engineer I think from Mesa Boogie who stated that they don't want any one to tinker (i.e. mess
with) with their amps so the bias is fixed. The theory is if you use the same grade of tube it will
draw the same amount of current so that no adjustment is necessary. Several tube companies provide
tubes that are matched (draw relatively the same amount of current) Groove tubes are matched for
current draw as well as for when the tube hits 10% distortion in order to create a more musical tube.
You can hear the difference.
Finally, it's great to see Ryan McMahon on the cover of this here mag. way to go Ryan.
Remember folks you heard about him here first.
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Website by: Guy Langlois - Webmeister/Publisher Cosmic Debris, 610 Lambert Ave, Nanaimo, BC, V9R 3N8